This invention relates to a system and a method for driving a nematic liquid crystal.
When two transparent flat plates having transparent electrodes and sandwiching a nematic liquid crystal are placed between two polarizing plates, transmittance of light passing through the polarizing plates changes with voltages applied to the transparent electrodes.
Since liquid crystal display devices based on the above principle can be shaped flat and are operative with low electric power, they have been widely used in wrist watches, electronic calculating machines, and so forth.
In recent years, they are also used in combination with color filters to form color display devices in note-type personal computers and small liquid crystal TV sets, for example.
There are some known types of dot matrix drive systems. A group of such systems is simple matrix drive systems having a simple structure. Another group is active matrix systems including TFT systems that can realize high-quality images by adding active elements to individual pixels.
Active elements are very difficult to make. Therefore, active matrix systems are expensive and need a large amount of investment for manufacturing facilities. However, they can use TN-type nematic liquid crystals that are advantageous for realizing high-quality images with a high contrast ratio, wide visual angle and multi-gradation.
Simple matrix drive systems have the merit that electrodes of liquid crystal panels can be made very easily. However, they involve the problem that the contrast ratio decreases as the duty ratio becomes high. Therefore, large-scaled matrix liquid crystal panels having a high duty ratio have been compelled to use STN-type nematic liquid crystals that are disadvantageous in contrast ratio, visual angle, response speed and multi-gradation.
In liquid crystal displays combined with color filters to display color images, three dots of different colors, namely, red, green and blue, are combined to display a desired color. However, color filters are very expensive and need a high accuracy when bonded to panels. Moreover, they need a triple number of dots to ensure an equivalent resolution as compared with black-and-white liquid crystal display panels. Therefore, liquid crystal color panels require a triple number of drive circuits typically in the horizontal direction. This means an increase of the cost of drive circuits themselves and the cost for an increased manhour for connecting drive circuits to the panel at a triple number of points.
That is, the use of color filters with liquid crystal panels to display color images involves many disadvantageous factors from the economical viewpoint.
To avoid the problems caused by the use of color filters, color liquid crystal display devices as disclosed in Japanese Patent Laid-Open 1-179914 (1989) have been proposed to display color images by combining a black-and-white panel and three-color back-lighting in lieu of color filters. Certainly, this method seems more likely to realize high-fidelity color images economically. Actually, however, because of the difficulty in driving liquid crystals at a high speed with conventional drive techniques, no such device has been brought into practice.
Another problem with conventional liquid crystal display devices is slow responses of liquid crystals. Due to this, liquid crystal display devices have been inferior to CRT displays especially when used as TV displays for displaying moving images or as personal computer displays required to follow quick movements of a mouse cursor.
Typical nematic liquid crystals have electro-optic characteristics substantially as shown in FIG. 1 in which the effective value of an applied voltage is material regardless of its polarities.
A driving method called active driving method has been proposed recently as one of driving methods using STN liquid crystal panels to realize a quality of images equivalent to that of TFT liquid crystal panels. That is, in order to improve the contrast ratio and the response speed, the active driving method relies on the approach that selects a plurality of scanning lines simultaneously to select scanning lines more often in each frame period. This is substantially the same as the conventional driving method in relying on the belief that the optical transmittance of a nematic liquid crystal exclusively depends on the effective value of an applied voltage.
Since nematic liquid crystals need time as much as decades of milliseconds to hundreds of milliseconds for response, it has been believed impossible to realize a speed of response acceptable for displaying color images by three-color back lighting.
The Inventor, however, has found that a specific status of applied voltage waveforms causes quick changes in optical transmittance with change in applied voltage level, while he measured dynamic characteristics of optical transmittance of nematic liquid crystals relative to waveforms of applied voltages for the purpose of developing a liquid crystal panel having a speed of response high enough to realize color images by three-color back lighting.
By using this phenomenon and by repeatedly generating the above-mentioned specific status, it has been made possible to drive nematic liquid crystals at a much higher speed with a higher contrast ratio than those by conventional drive techniques.
On the basis of the above knowledge, an object of the invention is to provide a new system and a method for driving a nematic liquid crystal which can increase the speed of response of any conventional nematic liquid crystals, either TN-type or STN-type, to a value high enough to ensure a performance equivalent to or higher than the performance of a CRT display system when displaying color images by the three-color back-lighting method or reproducing moving images.
Another object of the invention is to provide a matrix drive system and a matrix drive method of a nematic liquid crystal which realize both a high contrast ratio and a high response speed.
Another object of the invention is to provide a system and a method for driving a nematic liquid crystal which provides a high contrast ratio even in a large-scaled matrix liquid crystal panel having a high duty ratio and driven by the simple matrix drive system even when a TN-type nematic liquid crystal is used.
The invention is basically characterized in applying a voltage to a liquid crystal at a timing different from that of a conventional liquid crystal drive circuit to keep the contrast ratio high even when the duty ratio is high and to increase the response speed of the liquid crystal.
According to the present invention, there is provided a system for driving a nematic liquid crystal in a liquid crystal display device which includes a nematic liquid crystal, a plurality of common electrodes and a plurality of segment electrodes confining the nematic liquid crystal therebetween, and a pair of polarizing plates sandwiching the common electrodes and the segment electrodes confining the nematic liquid crystal, comprising:
means for applying a sequence of selection pulses to the common electrodes;
means responsive to the selection pulses to apply to the segment electrodes a voltage corresponding to image data to be displayed; and
means for applying to the segment electrodes a voltage different from the voltage corresponding to the image data in intervals where the selection pulses are not applied, the voltage applied to the segment electrodes being controlled such that the mean value thereof be a predetermined constant value.
According to another aspect of the invention, there is provided a method for driving a nematic liquid crystal in a liquid crystal display device which includes a nematic liquid crystal, a plurality of common electrodes and a plurality of segment electrodes confining the nematic liquid crystal therebetween, and a pair of polarizing plates sandwiching the common electrodes and the segment electrodes confining the nematic liquid crystal, comprising the steps of:
applying a sequence of selection pulses to the common electrodes;
in response to the selection pulses, applying to the segment electrodes a voltage corresponding to image data to be displayed;
applying to the segment electrodes a voltage different from the voltage corresponding to the image data in intervals where the selection pulses are not applied, the voltage applied to the segment electrodes being controlled such that the mean value thereof be a predetermined constant value.
In both aspects of the invention, the voltages to the common electrode and the segment electrode are preferably determined such that the voltage to the segment electrode be inverted in polarity when the selection pulse is applied to the common electrode.
The system preferably includes heater means for heating the nematic liquid crystal to a predetermined temperature.